The present invention relates to low-emissivity glazing, that is to say glazing which has the property of reflecting the infrared rays emitted, for example, by the inside of dwellings, and consequently limiting heat loss.
The demand for such glazing is often linked to that of having a light transmission as high as possible. The two demands of low emissivity and high transmission normally result in solutions that are opposing in terms of structure. It is necessary to carry out compromises which are difficult to establish.
The most common practice is to have systems of thin layers that comprise one or more layers capable of reflecting the infrared rays. Systems of this type are in general composed of metal layers, especially a silver layer with a thickness of a few nanometers. The layers must be thin enough not to reduce, in too significant a manner, the visible light transmission. The thickness must also be sufficient to block the transmission of infrared rays, the thickness directly determining the fraction of these rays which are effectively reflected.
The systems applied to glazing must simultaneously fulfill other conditions. Firstly, it is necessary to protect the reflective metallic layers against the chemical or mechanical attacks to which they may be exposed. The metal layers are normally deposited on the glass substrate by vacuum deposition techniques of the magnetically-enhanced sputtering type, more commonly known as magnetron sputtering. The layers obtained by these techniques offer the advantage of a high uniformity of the composition, thickness and surface finish. They are however very fragile and must be protected by additional layers. In a perfectly conventional manner, transparent dielectric layers made from metal oxides and/or nitrides, or else from mixtures of these, offering the required resistance, are used.
Simultaneously, the metal layers must also be protected from a possible diffusion from the substrate, a diffusion which would disadvantageously modify the properties of the reflective metal layer. The nature of the dielectric layers situated between the substrate and the metal layer is often similar to that of the layers located above this same metal layer. It is made from metal oxides and/or nitrides.
Conventionally, the sequence of the layers is composed in the following manner:
glass/dielectric I/metal/dielectric II
Among the most used dielectrics are, especially, ZnO, TiO2, SnO2, Si3N4, etc. These dielectrics offer various optical properties and are also distinguished by industrial production conditions.
The most common structures also incorporate one particular layer between the metal and the outer dielectric, a layer which has the role of protecting the metal, especially during the deposition of the layer of this dielectric. This is because, usually the formation of this dielectric is carried out in a “reactive” manner. Under these conditions, the dielectric (oxide or nitride) is formed at the same time as the deposition, by reaction of metal vapor emitted by bombardment of a metal cathode, with the atmosphere in which this deposition is carried out being at a very reduced pressure, for an oxide an atmosphere of oxygen or a gas mixture containing oxygen. Under these conditions, the deposited metal layer is in contact with this atmosphere and may be deteriorated, in particular due to the high reactivity of the plasma.
For protection against this deterioration, it is also common to have directly on the metal or suboxidized or nitrided layer reflecting the infrared rays, a so-called “barrier” or else “sacrificial” layer. This is a metal or partially oxidized layer of very small thickness, of which the role is to react with the constituents of the atmosphere which could deteriorate the metal layer that reflects the infrared rays. The barrier layer is carefully chosen both with regard to its nature and its thickness. It is not intended itself to be involved in the reflection of the infrared rays, but to react with the atmosphere in which the dielectric depositions of the layers deposited after that of the metal layer, that reflects the infrared rays, are carried out. To prevent this from substantially reducing the light transmission, it is important to ensure that the barrier layer on the one hand is as thin as possible, but more importantly, on the other hand, that it is practically completely converted into a transparent dielectric during its deposition or deposition operations following its own deposition.
Conventional systems consequently have the following succession of layers:
glass/dielectric I/metal/barrier/dielectric II
Where appropriate, the use of ceramic targets instead of metal targets avoids carrying out a reactive deposition. In other words, the oxide deposition may be carried out in an essentially inert atmosphere (as a general rule having less than 10% oxygen), thus avoiding the risk of oxidation of the previously deposited silver layer. In that case, it is possible to form a multilayer without a barrier layer on top of the silver.
The formation of these assemblies of layers must also result in colors that are satisfactory both in reflection and in transmission. The demand is for the most perfect neutrality possible. In the CIELab calorimetric coordinates, this corresponds to a* and b* values close to zero. Negative values, in particular for b*, are also acceptable. They give the glazing either a blue hue for negative values of b*, or a green hue for negative values of a*. Conversely, it is endeavored to avoid positive values of a*, which would result in purple and brownish hues.
The neutrality of the glazing depends on the choice of the combinations of layers. The layers forming the assemblies in question operate to form an interference system which makes it possible to remove the major part of the undesired wavelengths. The removal of these colors follows a mechanism that is well known in this field. The difficulty is in combining, at the same time, the calorimetric requirements with those linked to the “base” conditions: high light transmission and very low emissivity.
Furthermore, although the principles governing the optical properties of the materials forming the layers are well known, an additional difficulty comes from the techniques for producing this glazing. The deposition conditions, and especially the deposition rate, are dependent on the nature of the materials in question. For an economically acceptable industrial production, the rate must be sufficient. It depends on multiple factors which guarantee the stability of operation over time and over the entire surface of the sheet, and the absence of defects in the layer.